Electric attenuator



Jan. 3, 1950 A. c. BOWYER-LOWE ET Al. 2,493,653

ELECTRIC ATTENUATOR Filed Dec. l0, 1945 Patented Jan. 3, 195i) ELECTRIC `rrrTENUlrr-oa Albert C. Bowyer-Lowe and'Edmund R. Peach, Cambridge, England, assignors. to Pye Limited, Cambridge, England, a British company Application December 10, 1945, Serial No. :634,002 In Great Britain October 1'0, 1944 This invention relates to attenuators as used in telecommunication engineering for introducing a variable loss into a circuit without altering the other characteristics of the circuit. Attenuators normally consist of a number -of resistances connected in a manner to provide an attenuating network suitable for the purpose required. For use on high frequencies, a network is commonly employed which comprises `a plurality of similar sections connected in series, each section being composed of a series resistance and a shunt resistance connected in L-type -formation. With such `an Yarrangement each section reduces the power or voltage presented to fit from the preceding section, and by provision of a suitable switching or tapping-oit device in conjunction with the sections an attenuator can be constructed to provide, by operation of the switching or tapping device, a plurality of stages of attenuation progressively increasing insteps. Such an attenuator is commonly called a step attenuator, and it is to this kind cf attenuator that the present invention more particularly relates.

An object of the present invention is to provide an improved construction of a step attenuator, designed to reduce, as far as possible, stray or Adistributed capacity between the resistances, and between the resistances andthe usual earthed metal case of the device, and stray coupling between the input and output of the attenuator.

In a step attenuator according to the invention, screens .connected to the metal case of the device so as to 'be earthed therewith are provided in the case to partition it into compartments, the series resistances being arranged to pass through the 1screening partitions respectively, so that the ends oi' each series resistance are disposed in adjacent compartrnents at opposite sides of the intervening partitioning screen, and the shunt resistances being arranged Within said compartments respectively. With this arrangement, the ends of each series resistance are screened from each other, and the junction points of the series and shunt resistances of each section with the series resistance of the next section of the attenuator are disposed in separate compartments shielded by the partitioning screens.

In executing the described arrangement, it is most convenient to mount the resistances in the case oi the device so that the series vresistances are Substantially perpendicular to the shunt revscreening partitions, and the series resistances arranged .substantially horizontally and passing through the screensas described.

The .series resistances are preferably supported by their ends in clips which are mounted in the compartmentsand are insulated from, and spaced as far as possible from, adjacent vear-thed metal parts of the case and screening partitions. The shunt resistances are preferbly each supported at `one end by a `clip which is electrically .connected, preferably by direct mechanical mounting, with the clip or clips `.provided in the same compartment for the series resistances, the other end vof 4each shunt resistance being directly supported by the `case of the device so as thereby to be electrically connected thereto without interposed connecting wires or the like.

The caseof the device is provided ywitll-screened input and output sockets which are vdisposed so that the .conductors from them which are connected to the resistance network are disposed in separate compartments formed by the screening partitions, so that the input and output leads are eiectively-screened from each other.

The .clips lforming the junction points 'abovementioned are connected to suitable contact studs or the like arranged vto lie in the path of a movable pick-up contact which is adapted toimove over the studs and -i-s 'connected to an output lead for tapping oli stag-es of attenuation in steps as desired. Suitable locating means, such as springloaded detents or balls, maybe provided to ensure proper registration of the pick-up contact with the adjacent contact stud in eac'h position of adjustment of the pick-up contact. The pick-up contact is rpreferably furnished with a screen which moves with thecontact and screens it from all other contact studs or the like than the one which for the time being the pick-up contact is engaging.

In the preferred construction, which provides a compact device, the resistances are disposed in a circular arrangement in the-case which is provided with radial screening partitions, the contact studs or the like facing radially inwards in a circular array, and the pick-up contact is mounted to rotate within the circular .array of contact studs or 'the like for selectively engaging them.

A preferred embodiment; of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a cross-section of the device taken along the line fl---l yof Fig. 2.

Eig. 2 .is .a planntthe device with `the cover removed,

Fig. 3 is a fragmentary sectional view of a de tail, taken on line 3-3 of Fig. 2. and

Fig. 4 is a diagram of the attenuator network.

It will be convenient to refer rst to Fig. 4 which shows an attenuator network comprising ve similar sections connected in series and composed respectively of series and shunt resistances R1 and Re, Rz and Rv, R3 and Ra, R4 and R9, Rs and R10. The points T indicate the junction points previously mentioned and by arranging a pick-up contact P to travel along the points T, various stages of attenuation may be made available. For reasons unconnected with the present invention, the irst series resistance R1 of the network is provided in apparatus extraneous to the device presently to be described, and to indicate this the resistance R1 is shown in broken lines. Fig. 4 also depicts in developed form the arrangement of the resistances (R1 omitted) as employed in the device of Figs. 1 to .3 now to be described.

The device illustrated has a metal case comprising a base casting I and a main cover 2 the rim of which fits over-a spigot 3 formed on the outer Wall 4 of the base I and the top of which locates on the rim 5 of a recessed central boss 6 which upstands from the base I above the outer Wall 4, the cover 2 being secured to the rim 5 by screws l. The cover 2 thus closes the recess in the boss 6 to form a chamber 8 which is totally enclosed by metal. The cover 2 also forms with the base I an annular space surrounding the boss 6, which is totally enclosed by metal.

The annular space surrounding the boss 6 is partitioned into compartments 9 by radial webs I0 which extend between the boss Ii and the outer wall 4 and are formed integrally with the' base I. The partitions ID are of such height as to meet the top of the cover 2, and so completely separate the compartments 9 from one another. For an attenuator having five network sections as shown in Fig. 4, six partitions Ill forming six compartments are provided. In five of the compartments 9 there are xed to the boss 6 insulating blocks II which carry metal clips I2 supporting by their ends the cartridge-type series resistances R2, R3, R4 and R5 which are mounted horizontally in the case and pass through four of the partitions I0 respectively, so that the ends of each resistance are in separate compartments at opposite sides of a partition Ill, as shown more clearly in Fig. 2. For easily mounting the series resistances in this manner, the four partitions I0 concerned have their portions above the outer wall 4 provided with horizontal slots I3 (Fig. 3) on a level with the clips I2, so that the resistances can readily be slid sideways into the slots I3 and brought into engagement with their supporting clips I2. In order to complete the screening partitions, the slots I3 are afterwards closed by metal closing plates I4 (Figs` 2 and 3) which cover the open part of the slots I3 and are secured to the partitions IU by means of screws I5. As seen in Fig. 3, the blind ends of the slots I3 and the adjacent portions of the plates I4 are shaped to fit fairly closely around but not touching the-resistances.

The cartridge-type shunt resistances Rs, R1, Ra, R9 and R10 are mounted upright in separate compartments 9 with their lower ends supported directly on the bottom IS of the base I by being located in suitable recesses I1 in the bottom I6, the upper ends of these resistances being mounted iln metal clips I8 mounted on the insulating blocks The clips I2 and I8 in each compartment 9 rare secured to their insulating block II so as to ,be

electrically connected together, by means of a common contact stud I9 passing through the block and clips and fitted with a lock nut 29, the head of the contact stud I9 protruding into the chamber 8 through a hole 2I in the annular wall of the chamber 8. The hole 2l is made large enough for adequate insulation spacing of the stud I9 from the surrounding metal of the boss 6. In this way, a circular array of contact studs I9, providing the junction points T of Fig. 4, protrude into the chamber 8 for selective engagement by a rotary pick-up contact 22 provided in that chamber.

The rotary contact 22, in the form of an arcuate brush, is slidably mounted for parallel movement on a pair of parallel pins 23 fixed in an insulating enlargement or head 24 on the end of an insulating spindle 25 which passes axially through the boss 6 and projects from the bottom I 5 of the base I for attachment of a control knob or the like of its projecting end. The contact 22 is loaded by a spring 26 guided by a pin 27 xed in the head 25, so that the contact is urged outwards towards the annular wall of the chamber 9. The spring 2S is secured at one end, as by soldering, to the brush contact 22 and is conductively connected, for instance soldered, at its other end to a metal strip 28 fastened under a contact stud 29 screwed into the head 24, so that the contact 22 is electrically connected to the stud 29.

Fixed to the underside of the insulating head l 24 is a metal disc 30 which carries a metal shield 3| upstanding around the contact 22 as shown, so as to screen the contact 22 from all the contact studs I9 other than the one which the contact 22 is for the time being engaging. The undcrside of the disc 3U has V-grooves 32 or the like formed therein, and the boss t is bored to receive spring-loaded balls 33 which press against the underside of the disc 39 and are adapted to locate in the grooves 32 in the positions of engagement of the contact studs I9 by the contact 22, in order to ensure proper indexing of the contact 22 to its adjusted positions. The screen 3i is electrically connected to the case of the device, and thereby earthed, through the disc 3D and the balls 33. A suitable stop 34 is fixed to the annular wall of the chamber f5 to mark the end rotary positions of the contact 22, the stop 34 being engaged by the screen 3|.

It will be recalled that the resistances of the device occupy only ve of the six compartments 9. The stop 34 is mounted opposite the sixth compartment 9a over a hole 35 in the annular wall of the chamber a opposite that compartment. Electrical connection to the rotary contact 22 is made by a Contact 36 which rides on the stud 29 and is carried by a spring blade 3l Which passes with clearance through the stop 34 and hole 35 and is mounted on an insulating block 38 which is xed in the compartment 9a to the spigot 3 of the base I. Adjacent the compartment 9a, the base I is extended to form a tting for the output socket 39 of the device, the blade 3l being connected by a conductor `4i] to one output lead, the other output lead being connected to the socket 39 and thus to the base I. 'I'he output socket and leads are thereby effectively screened from the other parts of the device.

The input socket Il! is screwed into the outer wall 4 of the base I of the device adjacent the compartment 9 in which the first shunt resistance R6 is situated. One input lead is connected t0 the socket 4I and thus to the base I. The other input lead 42 is connected electrically to the first junction point T of the network, namely, the

junction between the resistances Re and Rz, the

the unused inlet being closed by a suitable plug 43. '.The input leads .are thus effectively screened from the output.

In the operation of the device, the pick-up contact 2-2 while being moved from one `contact stud I9 yto another is short-circuited to the case by its spring 26 causing the contact 22 to engage theannular wall of the chamber B. The purpose of this arrangement is to obtain silent adjustment oi the attenuator, to eliminate noise that would otherwise be produced during adjustment in other apparatus to which the attenuator may be connected.

By the improved screening obtained with the device described, an advantage results in that owing to the greatly reduced capacitance, much less input power is Arequired at high frequencies as compared with known constructions of step attenuators, and the accuracy of calibration of the device holds good over a wide range of irequencies, up to frequencies of the order of 100 megacycles, thus making the device especially valuable for obtaining accurately known extremely-small output powers or voltages as are used for 'the purpose Vof measuring the sensitivity of radio receivers and for other purposes in high frequency laboratories, and in the commercial manufacture of radio equipment.

We :claim:

1. .An electric `attenu'ator comprising a metallic case having internal metallic partitions forming a series of adjoining compartments within said case, va resistance network housed within said case comprising a vplurality of network sections connected in series, each section comprising a series resistor and a shunt resistor, the shuntresistors lof said network being disposed within said compartments respectively,lsaid partitions having vapertures therein respectively, the series `resistors of said network extending through said apertures nespect-ively in insulated relation to said partitions so that the ends of each series resistorY are disposed in adjacent Acompartments at the opposite sides vof the intervening partition between such compartments, whereby the ends of eachrseries resistorare .screened from each other and the junction points of the series and shunt resistors 'of leach network section with the series resistor of the next section of the network are disposed inseparate compartments shielded by said partitions, and a Aswitch mounted in said case for selecting sectionsA of said network to furnish -a .range of selectable stages of attenuation.

2. An electric attenuator as defined in claim 1, and comprising Isupport means in said -compartments for supporting the series and shunt resistors of said network substantially perpendicularly to each other.

3. An electric attenuator comprising a rmetallic case having internal metallic partitions forming a series of adjoining compartments within said case, a resistance network housed within said case comprising a plurality of network sections connected in series, each section comprising a series resistor and a shunt resistor, the shunt resistors of said network being disposed Within said compartments respectively, said partitions having apertures therein respectively, the series resistors offsaidnetwork extending through said apertures respectively in insulated relation to said partitions-.so that. the ends of each series resistor are disposed in adjacent compartments at opposite sides Aofthe intervening partition between such compartments, clip means mounted within said compartments in insulated and spaced relation to the walls thereof and supporting said resistors by their ends, whereby the ends of each series resistor arescreened from each other and the junction points. of the series and vshunt resistors of each network section with the series resistor of the .next section of the .network are disposed in separate lcompartments shielded by said partitions, and a switch mounted in said case for selecting sections of said network to furnish `a range of `selectable :stages of attenuation.

4. An electric attenuator comprising a metallic case having internal metallic partitions forming a series of'a'djoining compartments within said case, a network of :cartridge resistors housed within said case comprising a plurality of network :sections connected in series, each Section comprising a series resistor and a shunt resistor, theshunt resistors of said network being disposed within said compartments respectively, said partitions` having apertures therein respectively, the series resistors @of said `network extending through said apertures respectively. in insulated relation to saidV partitions so that the ends of each series resistor are disposed in adjacent compartments at the opposite sides ,of the intervening partition between-such compartments, means in each compartment supporting the shunt resistor therein at one end in direct electrical connection with said ca-se, support rneansl mounted in each ycompartment in insulated and spaced relation to the metallic Awalls thereof and including a Aconducting clip supporting the other end of the shunt -1 resistor inthe compartment and a conducting clip supporting the end of each series resistor extending into the compartment, a plurality of contactstuds respectively conductively connected to ysaid clips in Veach compartment and protrudingth-rough a wall of the compartment in insulated relation thereto, and a movable selector contact'mounted in said case for selectively engaging said. studs.

5. Anelectric .attenuator comprising .a metallic case-containing-a circular array of adjoining compartmenlts :separated by radial partitions, a plurality of contact studs .associated with individual compartments and mounted in insulated relation thereto to project radially inwards therefrom toj wards the .centre of said array, a rotary selector yContactinsulated from said case centrally mounted within .said array for selectively engaging said studs, a plurality lof clip assemblies mounted in individual compartments in conductive connection lwith the respective studs associated with said comprar rents and insulated from said case, a plurality of cartridge resistors individually in said compartments supported at one end by said clip assembly and at the other end in direct electrical connection with said case, said partitions having apertures therein and a plurality of cartridge resistors extending individually through said 'apertures between adjacent compartments and each supported at its ends by the respective clipassemblies in such. adjacent compartments, all said resistors together constituting a network of series resistors constituted by said resistors extending between adjacent compartments and shunt resistors constituted by said resistors I mounted within the individual compartments.

6. An electric attenuator as defined in claim 5. and further comprising a metallic shield extending about said contact and mounted to move therewith.

7. An electric attenuator as dened in claim 5, wherein said selector contact is adapted to be short-circuited to said case when not engaging one of said studs.

' 8. An' electric attenuator as defined in claim 5, and further comprising a screened input socket and a screened output socket mounted on said case in locations such that input and output condu-ctors passing through said sockets respectively enter different compartments within said case whereby to shield the input and output conductors of the attenuator from each other.

9. An electric attenuator of the class described comprising a metallic case having a base and upstanding concentric annular outer and inner walls forming a chamber within said inner wall and an annular space between said inner and outer walls, and having metallic walls extending radially between said inner and outer walls to partition said annular space into a series of adjoining compartments screened from one another, a plurality of clip assemblies in individual compartments and mounted on said inner wall and insulated therefrom, each clip assembly including a contact stud and said inner wall having apertures therein through which said contact studs respectively protrude into said chamber in insulated relation to said inner wall, a plurality of cartridge resistors disposed upright within individual compartments and each supported at its lower end by Said base so as to be in direct electrical connection therewith and each supported at its upper end by said clip assembly in the compartment, said radial partition walls each having an aperture therein, a plurality of cartridge resistors disposed horizontally in the individual apertures in said radial walls so as to each extend into adjacent compartments and each being supported at its ends by the respective clip assemblies in such adjacent compartments, whereby to form a resistance network comprising a plurality oi network sections connected in series, each section comprising a series resistance constituted by said horizontal resistor and shunt resistance constituted by said upright resistor in a compartment, the ends of each series resistor being screened from each other by the intervening partition wall and the junction points of the series and shunt resistors of each section of the network with the series resistor of the next section of the network being disposed in separate compartments, and a rotary selector contact assembly axially mounted within said chamber in insulated relation to said case for selectively engaging said contact studs protruding into said chamber, means axially extending from said case for rotating said selector contact assembly, and conductor means insulated from said case and leading from said selector contact assembly to the exterior or said case without entering any compartment containing resistors.

10. An electric attenuator as defined in claim 9, wherein said rotary contact assembly includes a contact mounted for radial movement and spring means urging said contact radially outwards so as to engage with said inner Wall and be thereby short-circuited to said case when not engaging one of said contact studs.

11. An electric attenuator as defined in claim 9, wherein said rotary contact assembly includes a contact for selectively engaging said studs and a metallic screen extending about said contact and rotatable therewith to shield said contact yfrom all other of said contact studs than the one which said contact selectively engages.

12. An electric attenuator as defined in claim 9, wherein said rotary contact assembly includes a contact mounted for radial movement and spring means urging said contact radially outwards so as to engage with said inner wall and thereby be short-circuited to said case when not engaging one of said contact studs, and a metallic shield rotatable with said contact and extending thereabout to shield said contact from all other of said contact studs than the one which said contact selectively engages.

13. An electric attenuator as defined in claim 9, .wherein said annular space includes a compartment free from resistors, and wherein the attenuator further comprises an output socket housing mounted on said base beyond said outer Wall and adjacent said compartment, said outer wall having an aperture therein affording communicating between said housing and said compartment, and an insulating mount xed within said compartment and supporting a conductor extending through an aperture in said inner Wall to make electrical connection with said rotary contact assembly, the supported end of said conductor within said compartment being electrically connected with a lead extending through said aperture in said outer wall into said socket housing.

14. An electric attenuator as dened in claim 9, wherein said annular space includes a compartment free from resistors, and wherein the attenuator further comprises an output socket housing mounted on said base beyond said outer wall and adjacent said compartment, said outer wall having an aperture therein affording communicating between said housing and said compartment, and an insulating mount fixed within said compartment and supporting a conductor extending through an aperture in said inner wall to make electrical connection with said rotary contact assembly, the supported end of said conductor within said compartment being electrically connected With a lead extending through said aperture in said outer wall into said socket housing, said outer wall having an aperture therein opposite the compartment housing the first resistor of the netwerk for attachment of an input socket thereto.

ALBERT C. BOWYER-LOWE.

EDMUND R. PEACH.

REFERENCES CITED UNITED STATES PATENTS Name Date Van Beuren June 9, 1942 Number 

